Included in the background of the present invention are technologies relating to heterojunction bipolar transistors (HBTs, which are electrical tilted-charge devices) and light-emitting transistors, transistor lasers, and tilted-charge light-emitting diodes (respectively, LETs, TLs, and TCLEDs, all of which are optical tilted-charge devices). A tilted-charge device gets its name from the energy diagram characteristic in the device's base region, which has, approximately, a descending ramp shape from the emitter interface to the collector (or drain, for a two terminal device) interface. This represents a tilted-charge population of carriers that are in dynamic flow—“fast” carriers recombine, and “slow” carriers exit via the collector (or drain).
Regarding optical-tilted charge devices and techniques, which typically employ one or more quantum size regions in the device's base region, reference can be made, for example, to U.S. Pat. Nos. 7,091,082, 7,286,583, 7,354,780, 7,535,034, 7,693,195, 7,696,536, 7,711,015, 7,813,396, 7,888,199, 7,888,625, 7,953,133, 7,998,807, 8,005,124, 8,179,937, 8,179,939, and 8,494,375; to U.S. Patent Application Publication Numbers US2005/0040432, US2005/0054172, US2008/0240173, US2009/0134939, US2010/0034228, US2010/0202483, US2010/0202484, US2010/0272140, US2010/0289427, US2011/0150487, and US2012/0068151; and to PCT International Patent Publication Numbers WO/2005/020287 and WO/2006/093883, as well as to the publications referenced in U.S. Patent Application Publication Number US2012/0068151.
Applicant has considered the effects of quantum wells in the base region of a high speed electrical tilted-charge device (HS-ETCD), such as a high speed heterojunction bipolar transistor (HBT). An experimental study indicated that a large amount of minority carrier charge is stored within the quantum well region of the device. The increased charge storage in the base region containing the quantum well then resulted in a lower cutoff frequency (ft) and a lower electrical gain-bandwidth product.
For example, in FIG. 2 characteristics of two high speed devices are compared. The first device is a traditional high speed electrical device, in this example a high speed electrical tilted-charge device, such as the heterojunction bipolar transistor (HBT) shown in FIG. 1. In the example of the FIG. 1 device, the layers are III-V semiconductor materials including an undoped substrate 110, an n-type sub-collector layer 120, an n-type collector layer 130, a p-type base layer 140, and an n-type emitter layer 150. Metal contacts shown at 131, 141 and 151 comprise the collector, base, and emitter electrodes, respectively, and the potentials at these electrodes are designated VC, VB, and VE, respectively. An example of DC bias range conditions is VBE>1.2 volts and −2.5 volts<VBC<0.5 volts.
The second device of this example is similar, except for the presence of two quantum wells (not shown) embedded in the base region of the device. The second device will have enhanced carrier recombination in the quantum wells of the base region. Both devices of the example have emitter mesa dimensions of 4.2 um by 7.4 um, and were biased with the same emitter current of 4 mA.
FIG. 2 shows a plot of current gain as a function of frequency for the two devices. The cutoff frequency (fc) is defined as the frequency where the ac current gain (βac=IC/IB) equals 1. For this example, the high speed electrical tilted-charge device (HS-ETCD) with no quantum well has a cutoff frequency (ft) of 10.4 GHz and a 3 dB frequency of 0.24 GHz. On the other hand, the high speed optical tilted charge device (HS-OTCD) containing two quantum wells embedded in the base region has an ft of only 1.3 GHz and of 3 dB frequency of 0.42 GHz. The large reduction of the cutoff frequency, ft, for the high speed optical tilted charge device (HS-OTCD) makes the device unsuitable for most electrical applications traditionally associated with an HS-ETCD (e.g. drivers, amplifiers, or oscillators). Furthermore, the electrical gain-bandwidth product of an HS-OTCD is substantially temperature dependent due to quantum well re-thermalization effects.
FIG. 3 shows the optical bandwidth cutoff (foc) of the high speed optical tilted charge device (HS-OTCD). The optical bandwidth cutoff (foc) is defined as the frequency where the optical power to detector power sensitivity ratio is 1. As indicated, the corresponding HS-OTCD above has an optical bandwidth cutoff (foc) of ˜10 GHz despite having an electrical cutoff frequency of only 1.3 GHz.
It is accordingly evident that the potential use of a high speed tilted-charge device as both an optical device and an electrical device is substantially limited by the effects described above. It is among the objects hereof to address these limitations of the prior art and to set forth devices and techniques for dual mode operation of high speed tilted-charge devices such as light-emitting transistors.